168: IRF8 and IRF3 cooperatively regulate rapid interferon-β induction in human blood monocytes

Abstract

Type I IFN production differs in magnitude and kinetics between different cell types. Robust and rapid induction of interferon-β (IFN-β) after pathogenic stimulation is a key property that is exclusive to human blood monocytes. This disparity raised the possibility that distinct myeloid-specific transcription factor(s) may be involved in the rapid induction of IFN-β in monocytes compared with non-myeloid cell types. We found that IFN-β was produced rapidly in primary human monocytes as a result of cooperation between the myeloid-specific transcription factor IRF8 and the ubiquitous transcription factor IRF3. We provide evidence that IRF8 constitutively binds to the IFN-β promoter region, and knockdown of IRF8 in monocytes abrogated IFN-β transcription. We uncovered a requirement for IRF3, a master regulator of IFN-β production, as a previously unidentified interaction partner of IRF8. We produced a range of deletion constructs of IRF3 and IRF8 mutants, and, using co-transfection and co-immunoprecipitation, mapped the protein–protein interacting regions of IRF3 and IRF8, and found that their interaction was independent of the DNA-binding domain (DBD) and the IRF association domain (IAD) of IRF8 and IRF3, respectively. Through these and other experiments, we have been able to demonstrate that IRF8 directly synergizes with IRF3 in monocytes to facilitate faster IFN-β transcription after pathogenic stimulation.